The present invention pertains to type I dielectric compositions that frit at high temperatures, especially type I dielectric compositions based on neodymium titanate.
Dielectric compositions having useful properties for making capacitors have been classified according to various criteria such as their temperature coefficient (defined as the product of the relative variation of capacitance and the converse of the temperature variation ##EQU1## and their dielectric constant. Thus type I dielectric compositions are those that have a low dielectric constant, ranging notably between 10 and 90.
Prior art type I dielectric compositions notably exhibit a dielectric constant temperature coefficient which is practically zero within very wide range of temperatures. They also display very low dielectrical losses (generally smaller than 6.10.sup.-4) at high frequences of greater than 1 MHz and over a very wide range of temperature between -55 degrees C. and +125 degrees C., when materials such as lead titanate and neodymium titanate. This type of composition is generally used to make either disk-shaped or multi-layered ceramic capacitors. Now, multi-layered ceramic capacitors are generally made by casting or shaping layers of dielectric, depositing conductive metallic electrodes on the insulating layers, stacking the resultant elements to form a multi-layered capacitor and fritting the material at a high temperature to densify it and form a solid structure.
Multi-layered ceramic capacitors have various applications and the specialist knows that the presence of bismuth in a type I dielectric ceramic composition does not enable capacitors of this type to be used at high frequencies. It is therefore indispensable to eliminate the bismuth from any type I dielectric composition. Bismuth, however, has the advantage of reducing the fritting temperatures of a ceramic.
An object of the present invention, therefore, is to remove these disadvantages by proposing a new type I dielectric composition which frits at high temperatures, this composition being chosen from the following diagram: EQU Nd.sub.2 O.sub.3.3TiO.sub.2 -PbTiO.sub.3 -BaTiO.sub.3 -BaZrO.sub.3
while at the same time preserving the performances of capacitors that use type I dielectric compositions. For a frequency of 1 MHz, these performances are the following:
Dielectric constant ranging from 75 to 85, PA1 Dielectric loss factors of 4 to 8.times.10.sup.-4 which are stable at high frequency, PA1 A temperature coefficient .alpha. varying from 0 to .+-.30 ppm corresponding to the NPO class.